How Your Smartphone Light Sensor Could Help Websites Track You

A researcher details the theoretical privacy risks created by your smartphone light sensor.

If you have a smartphone, chances are it's able to tell how dark or bright its surroundings are.

Thanks to the phone's light sensor, advertisers, online trackers, and perhaps even law enforcement might soon have a chance to use that feature to track and profile you, according to a privacy and security researcher who warned of the potential dangers posed by ambient light sensors.

In a blog post published on Wednesday, Lukasz Olejnik analyzed how a smartphone or laptop's light sensors could be used to fingerprint a user and reveal information about him or her. In theory, Olejnik posited, it could even be possible to map a building or home based on the light detected by the phone.

"If it's possible to map the user's home arrangement, this information could be potentially possible to discover its size, number or rooms, etc," Olejnik, a London-based security and privacy consultant and a researcher at University College London, wrote. "Such information is related to the user's financial situation, and consequently it would lead to a profile the user—allowing to assign him to a particular category such as 'this user has a large house, he is wealthy.' Why not target web content based on this information?"

"Browsers are now becoming so complicated and feature rich that it's very hard to predict how adversaries might use this stuff to impact privacy."

Web browsers and websites will soon be able to access the information gathered by your devices' light sensors thanks to the World Wide Web Consortium (W3C) new Ambient Light Sensors API, which is already effectively deployed in Firefox and coming soon on Chrome and possibly Safari.

Tapping into this data, it will be possible to "profile, detect, recognize and track" users and their behavior, such as what time they usually work, what lighting conditions they prefer, and how frequently they are in their house or office, according to Olejnik.

"Sensors data is privacy sensitive, but the actual nature of the data is not yet entirely known," Olejnik told me in an email, adding that there's a need for more research into this domain, which is why he launched the project SensorsPrivacy.com.

Other ways to use light sensor data to track users, he added, would be to link several devices with a single user, and detect the presence of people in a same room based on the ambient light.

In general, this might indicate a trend toward more tracking through the browser, in ways we can't even imagine yet.

"Browsers are now becoming so complicated and feature rich that it's very hard to predict how adversaries might use this stuff to impact privacy," the technologist at the Center for Democracy and Technology Joseph Lorenzo Hall told me, adding that people who work at W3C on privacy, such as himself and Olejnik, "are increasingly concerned about the chattiness of sensors and newer HTML APIs."

For Olejnik, browsers and technologies in general should have privacy built in by design, which is why he is studying the unintended privacy dangers of these new data sources. Until browsers have better privacy baked in, if you're worried about your chatty sensors and you're using Firefox, you can disable any sensors in "about:config" by setting "device.sensors.enabled" to "false" by double clicking on it.